Conventionally, inversion-mode transistors have been widely used in semiconductor devices such as ICs and LSIs. In an inversion-mode transistor, the directions of effective electric fields applied to a gate insulating film and a channel region are the same, respectively, in the on and off states of the transistor. This is because, in order to turn on the inversion-mode transistor, it is necessary to induce charges of the same sign as that of carriers into a depletion layer until the depletion layer reaches the maximum depletion layer width, thereby forming a channel for the carriers to move therein. Accordingly, for enhancing the driving capability of a drain current (the drain current driving capability) when the transistor is in the on state, it is effective to set a voltage applied to a gate electrode as high as possible. However, in consideration of the reliability of the gate insulating film, the absolute value of an electric field that can be applied to the gate insulating film is determined to be, for example, 8 MV/cm or the like and therefore there has been a problem that, as a result thereof, the voltage swing applied to the gate electrode cannot be set large and thus the current-driving capability of the transistor cannot be enhanced.
Herein, if it is possible to enhance or improve the drain current driving capability that increases quadratically with respect to a value of difference, “gate voltage VG” minus “threshold voltage Vth,” as generally given by the following formula (1), the operating speed of a circuit can be enhanced or improved.
                              I          Dsat                =                              1            2                    ⁢                      W            L                    ⁢                      C            i                    ⁢                                    μ              ⁡                              (                                                      V                    G                                    -                                      V                    th                                                  )                                      2                                              [                  Formula          ⁢                                          ⁢          1                ]                            where IDsat is a saturated drain current, W and L a gate width and a gate length, Ci a unit capacitance of a gate insulating film, μ a mobility of an electron or a hole, VG a gate voltage applied to a gate electrode, and Vth a threshold voltage. The fact that the voltage swing applied to the gate electrode cannot be set large represents that “gate voltage VG” minus “threshold voltage Vth” cannot be set large, and therefore, as a result thereof, there has been a limit to the improvement in drain current driving capability and the improvement in the operating speed of a circuit.        
On the other hand, accumulation-mode transistors are also known. However, a conventionally proposed accumulation-mode transistor is configured to realize normally-off by reducing the impurity concentration of a channel region to less than 2×1017 cm−3. In the channel region of such an accumulation-mode transistor, a current amount flowing in a semiconductor substrate becomes sufficiently small as compared with a current amount flowing in an accumulation layer near the interface between a gate insulating film and a semiconductor and thus on/off cannot be controlled by the current flowing in the semiconductor substrate. Since it is necessary to sufficiently induce carriers into the accumulation layer for turning on the transistor, the directions of effective electric fields applied to the gate insulating film and the silicon channel become the same, respectively, when the voltage applied to a gate electrode is at a threshold value representing the boundary between on and off and after the transistor is turned on, and thus the voltage swing applied to the gate electrode cannot be set large, similarly to the inversion-mode transistor.